The use of television is commonplace in the United States and throughout the world. Nearly every home in the United States has at least one television set. Many homes have cable television, which couples a large number of television channels to the home through a single coaxial cable. Other homes and businesses may have satellite receivers that are capable of receiving television signals from a number of satellites in stationary orbit around the earth.
Television signals are defined by the National Television Standards Committee (NTSC). Each television signal comprises a video signal and an audio signal. The NTSC signal, which evolved when only black and white (B/W) television was available has a baseband bandwidth of approximately 4.7 megahertz (MHz). The NTSC signal is modulated to a predetermined carrier frequency. For example, VHF channel 2 has a carrier frequency of 55.25 MHz. A small spacing in the frequency spectrum between adjacent channels prevents interference between channels. The bandwidth of the modulated signal is approximately 6.0 MHz. Other transmission systems, such as cable broadcasting, may use different frequencies for the television channels.
When color television was introduced, it was important that the color signals be added in a manner that did not interfere with the normal operation of B/W television signals. This was accomplished by introducing a chrominance signal modulated at a frequency that causes the chrominance signal for each line of the television signal to have an inverted phase with respect to the prior line. There are an odd number of lines in each television frame, with the result being that the chrominance signal for any given line is inverted in alternating frames of the television signal. The phase inversion causes the chrominance signal to cancel out temporally over the time of one frame, and spatially in the vertical axis over the space of two lines. The cancellation prevents the chrominance signal from erroneously being interpreted as part of the luminance signal. This effect, combined with the known persistence of vision in humans causes the chrominance signal to effectively cancel out in a B/W television so that it causes no noticeable interference. The NTSC signal has a modulated chrominance signal that overlaps the luminance signal in a portion of the frequency spectrum where the overlap causes minimal interference.
The frequency spectrum of the NTSC signal is shown in FIG. 1A. As can be seen in FIG. 1A, the video signal comprises a luminance signal 2 and a chrominance signal 4. The luminance signal 2 provides the signal intensity for both B/W and color television signals. The luminance signal 2 has spectral peaks 6 every 15.75 kilohertz (kHz), which corresponds to the horizontal frequency in the television. The amplitude of the luminance spectral peaks 6 decreases up to 4.2 MHz. The video signal is suppressed above 4.2 MHz to permit the insertion of an audio signal 5 in the spectrum for the particular video channel. The audio signal 5 is modulated with a 4.5 MHz carrier.
The chrominance signal 4 is introduced beginning at about 2 MHz in the spectrum. The chrominance signal 4 has chrominance spectral peaks 8, which are also spaced 15.75 MHz apart in the frequency spectrum. The chrominance signal is modulated at a frequency of 3.579545 MHz (an odd multiple of half the line scan frequency) to cause the chrominance signal peaks to interlace with the luminance peaks, as shown in FIG. 1B, which illustrates a magnified portion of the spectrum of FIG. 1A.
As seen in FIG. 1B, the luminance spectral peaks 6 and the chrominance spectral peaks 8 are spaced apart by 7.875 kHz. Although FIG. 1B, shows the frequency spectrum with no overlap, there is some degree of overlap in these signals due to the non-periodicity of the signals with respect to the line scan frequency.
The NTSC signal has temporal characteristics as well as the frequency characteristics described above. A single video frame comprises 525 video lines that are displayed in two interlaced video fields. Each video field is displayed with a vertical display rate of approximately 60 Hz (59.94 Hz) so that a video frame (with two interlaced video fields) is displayed at a vertical display rate of approximately 30 Hz (29.97 Hz). As seen in FIG. 2, there are two luminance peaks L1 and L2, spaced apart in the frequency spectrum by 30 Hz. The chrominance signal 4 is inserted between alternating pairs of luminance peaks 6. If one selects an arbitrary luminance peak L1 as a reference luminance peak, it is readily seen that the chrominance signal 4 has a spectral peak C 15 Hz above the reference luminance line peak L1. A second luminance peak L2 is spaced 15 Hz above the chrominance peak C (and 30 Hz above the reference luminance peak L1 ). The luminance peak L1 appears again 60 Hz above the reference luminance peak L1. Thus, the pattern repeats every 60 Hz. It should be noted that there is no signal in the spectrum 45 Hz from the reference luminance peak L1. As described in the prior art, that spectral "hole" in the spectrum is currently unused, and could carry additional information. The frequency spectrum of the NTSC signal with additional information signal D added is shown in FIG. 3. Note that the additional information signal is added to an unused portion of the spectrum that, in an ideal case, will cause no interference with the normal video signal processing.
The use of this spectral hole is described in U.S. Pat. No. 4,660,072, which is incorporated herein by reference. The patent describes a technique for adding an additional luminance signal to a standard video signal by inserting the additional luminance signal into the unused portion of the spectrum. The system disclosed in the patent modulates a high frequency luminance signal with a 3.579545 MHz carrier that abruptly switches phase every field of the NTSC signal (60 Hz). The carrier signal is thus modulated by a 30 Hz square wave that has alternating phases of the carrier signal.
The selected carrier frequency and alternating phases cause the additional luminance signal to cancel out temporally and spatially in the same manner as the chrominance signal. The additional luminance signal ideally averages to zero, but in reality the signal averages to zero only if it is unchanging over time. Thus, the additional luminance signal will completely cancel only if it is unchanging. In signal processing terms, only common mode signals are completely canceled. Differential signals do not cancel each other out and will remain in the NTSC signal as a residual signal that may cause interference with the luminance signal. The amount of residual signal depends on the bandwidth of the additional luminance signal and the correlation of the additional luminance signal with the NTSC standard luminance signal. The greater the bandwidth of the additional luminance signal, the greater the amount of additional luminance signal that will feed through and become visible to the television viewer (in the form of interference). In addition, the less correlation between the additional luminance signal and the NTSC standard luminance signal, the greater the amount of additional luminance signal that will feed through and become visible to the television viewer in the form of interference.
The selection of a 30 Hz square wave as a modulation source creates additional problems not solved by the system described in the U.S. Pat. No. 4,660,072. Because an ideal square wave contains an infinite number of odd harmonics, the additional luminance signal is modulated not only at 30 Hz, but at all odd harmonics of these two signals as well. The modulation by many multiple frequencies increases the possibility that the additional luminance signal will overlap in the frequency domain with the video signal. The overlap with the video signal may not present a significant problem in the application described in the patent because the additional luminance signal is highly correlated with the NTSC standard luminance signal, so the interference may not be noticed by the viewer.
However, if the additional information signal added to the standard video signal is unrelated to the video signal, the approach disclosed in U.S. Pat. No. 4,660,072 may be unsuitable because the interference with the video signal may be intolerable. Furthermore, there may be unacceptable interference for the additional information signal itself. To avoid interference, it is necessary to reduce the bandwidth of the additional information signal. There is theoretically a 1.8 MHz bandwidth available in the unused portion of the chrominance spectrum. Because standard modulation creates two sidebands, the actual data bandwidth is limited to 0.9 MHz. The modulation technique proposed in U.S. Pat. No. 4,660,072 causes an unacceptable spectral spreading of the additional information signal that can cause interference with normal television operation.
Therefore, it can be appreciated that there is a significant need for a system and method for introducing an additional information signal into a video signal without the undesirable effects of signal interference or reducing bandwidth to avoid interference.